Electronic sound synthesizer

ABSTRACT

An electronic sound synthesizer which includes a touch controlled voltage generator for selectively controlling the generation of a plurality of electrical signals and means for converting the signals into audible sounds. The generator includes a plurality of electronic switches in series with variable resistors, a direct current current summing amplifier and a plurality of conductive paths for selectively connecting the switches to the d.c. current summing amplifier. Each of the conductive paths has a pair of flat conductor members spaced from each other by a dielectric member. These conductor members are designed so that when a finger touches the members of a pair a conducting path is formed to energize the corresponding switch. The synthesizer may also include means for modulating and filtering the output signals of voltage sensitive wave generators controlled in turn by other wave generators or by the touch controlled voltage generator, or both. It also includes means for modifying the attack and decay amplitude characteristics of the filtered output signals before they are converted into sounds. The converting means may include a mixer, a reverberator, a tone control circuit, a power amplifier, and a speaker, all of which are interconnected and may combine the output signals of the various touch controlled generators. Alternatively, the synthesizer may have means including a plurality of ringing circuits, instead of the electronic switches and variable resistors which may be selectively actuated to generate audible sounds of various predetermined characteristics by touching corresponding conductive paths.

[451 Jan. 2, 1973 [54] ELECTRONIC SOUND SYNTHESIZER [76] Inventors: Charles Keagle, 312 East 9th Street,

Apt. 6, New York, N.Y. 10003; Alan Waggoner, 1512 East McGraw Street, Seattle, Wash. 98102; Peter Primary Examiner-Lewis H. Myers Assistant Examiner-U. Weldon Att0rneyTh0mas B. Graham [57] ABSTRACT An electronic sound synthesizer which includes a touch controlled voltage generator for selectively controlling the generation of a plurality of electrical signals and means for converting the signals into audible sounds. The generator includes a plurality of electronic switches in series with variable resistors, a direct current current summing amplifier and a plurality of conductive paths for selectively connecting the switches to the dc. current summing amplifier. Each of the conductive paths has a pair of flat conductor members spaced from each other by a dielectric member. These conductor members are designed so that when a finger touches the members of a pair a conducting path is formed to energize the corresponding switch. The synthesizer may also include means for modulating and filtering the output signals of voltage sensitive wave generators controlled in turn by other wave generators or by the touch controlled voltage generator, or both. It also includes means for modifying the attack and decay amplitude characteristics of the filtered output signals before they are converted into sounds. The converting means may include a mixer, a reverberator, a tone control circuit, a power amplifier, and a speaker, all of which are interconnected and may combine the output signals of the various touch controlled generators. Alternatively, the synthesizer may have means including a plurality of ringing circuits, instead of the electronic switches and variable resistors which may be selectively actuated to generate audible sounds of various predetermined characteristics by touching corresponding conductive paths.

31 Claims, 15 Drawing Figures 'roucn CONTROLLED CAIOEAOiEED N GENERATOR AMPLIFIER (FI -2) (FIGS) 1 V WAVE WAVE GENERATOR GENERATOR A 5 (F165). (F164) (no.5)

DRUM GENERATOR X REVER- TONE POWER E BERATOR CONTROL AMPLIFIER 423 Q R SPEAKER MICROPHONE g9.-

42/ MICROPHONE PRE AMPLIFIER PATENTED 2 I975 sum 03 0F 13 PETER PHJLL IPS PATENTEDm 2 I975 SHEET 0 [1F 13 NVENTORS CHARLES KEAGLE ALAN FE'I'E' I,

GONER HILLIPS ATTORNEY mdE EOE PATENTED AI 2 7 SHEET O8Uf13 ZOKHZ VOLTAG E DURATION OF TOUCH ON KEYBOARD UNIT TIME TIME

U U U U U n n n n H II TIME TIME

TIME

TIME

INVENTORS CHARLES KEAGLE ALAN WAGGONER I 3%:ER PHILLIPS PATENTEDJAN 2 I975 I I SHEET 090F'13 INVENTORS CHARLES KEAGLE ALAN WAGGONER PETER PHILLIPS ATDI'ORNEY PATENTEDJAN 2197s SHEET 110F113 INVENTORS CHARLES KEAGLE ALAN WAGGONER PETER PHlLLIPS ETTORNEY INVENTORS IHARLES KEAGLE S753 QT min WAGGONER BE'TERPHILLIPS ATTORNEY ELECTRONIC SOUND SYNTHESIZER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electronic instruments, for generating audible sounds generally and,more particularly, to an electronic sound synthesizer for generating orchestrated and other sounds of different characteristics.

2. Brief Description of the Prior Art Electronic instrumentation to generate sounds which are similar to those produced by more generally known means such as the human voice and conventional musical instruments is a relatively recent innovation. Generally speaking, available electronic sound generators are complex and expensive.

BRIEF DESCRIPTION OF THE INVENTION It is therefore an object of the present invention to provide a simplified and relatively inexpensive electronic system for generating sounds of certain characteristics in general and, more particularly, orchestral sounds of different natures.

It is another object of the present invention to provide an electronic system which can produce and synthesize sounds having different characteristics.

It is a further object of the present invention to provide the combination of an electronic sound synthesizer and novel electronic distribution means for providing plurality of audible outputs.

It is still another object of the present invention to provide means for modifying the tonal character of the electronically generated sounds.

These and other objects of the present invention are achieved by providing an electronic sound synthesizer which includes one or more generators, each designed to generate voltages of predetermined amplitudes, and means for converting the voltages into audible sounds. The generator is provided with a dc. voltage source, a plurality of electronic switches, and a plurality of conductive paths for selectively connecting the switches to the dc. voltage source. Each path includes a relatively flat conductor pair, spaced by a dielectric means of such a dimension that the space can be readily bridged over by a fingertip to short the path and thereby energize the corresponding switch. The switch provides a selective current path through a variable resistor to a summing point at the input of a current summing amplifier. The output of this amplifier provides variable potentials which are used to control voltage sensitive wave generators. The plurality of conductive paths may be conveniently mounted on a dielectric board and aligned in the manner of a conventional musical keyboard.

The wave generators are designed to provide wave forms of desired characteristics. The wave forms are modulated and filtered to produce outputs of certain desired characteristics. The converting means includes a mixer, reverberator, tone control circuit, power am-' plifier and speaker interconnected for converting the electrical signals produced by one or more of the sound generators into audible sounds.

It is a feature of the present invention to provide one or more auxiliary wave generators for generating a train of waves whichare used to modulate the output of other wave generators as described above and a filter tion;

It is still another feature of the present invention to I provide a synthesizer having a plurality of oscillators for generating a plurality of percussive sounds such as those of drums.

It is another feature of the present invention to provide a plurality of ringing circuits in a touch controlled generator of a design that can operate as a multi-voice rhythm band.

It is a further feature of the present invention to provide wave generators, ringing oscillators, filters and voltage controlled amplifiers uniquely designed to enhance the overall function of the electronic sound synthesizer.

It is still another feature of the presentinvention to provide a light controlled multi-channel distributor for distributing the outputs of the synthesizer to different channels in a predetermined sequence or sequential set.

The aforementioned and other objects and features of the present invention will become more apparent from the following detailed description of the present invention and the accompanying drawings showing several embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic block diagram of an embodiment of the electronic sound synthesizer in accordance with the present invention;

FIGS. 2 through 6 show in detail various components of the block diagram of the synthesizer shown in FIG. 1 in which the figures detailing the blocks are parenthetically identified, wherein;

FIG. 2 shows in detail a novel touch controlled voltage generator in accordance with the present inven- FIG. 3 shows a wave generator designed to generate any combination of trains of square waves and sawtooth waves either of a fixed frequency or in response to the touch controlled generator shown in FIG. 2;

FIG. 4 shows a second wave generator designed to generate any combination of trains of square waves, sawtooth waves and triangular waves in response to the outputs of the first wave generator shown in FIG. 3 and the touch controlled generator shown in FIG. 2;

FIG. 5 shows filter responsive to the output of the touch controlled generator and the output of the second wave generator shown in FIG. 4;

FIG. 6 shows a voltage controlled amplifier responsive to the outputs of the touch controlled generator, and the filter for modifying the transient amplitude characteristics of the generator output;

FIGS. 7 and 8 shows wave forms at various locations in the synthesizer shown in FIGS. 2 through 6;

FIG. 9 shows another embodiment of the present invention including a touch controlled generator for producing percussive sounds similar to those of drums;

FIG. 10 shows wave forms at certain locations of the circuit of FIG. 9;

FIG. 11 shows still another embodiment of the present invention including a touch controlled generator designed to produce a multi-voice rhythm band;

FIG. 12 shows in detail a twin-T oscillator, a plurality of which may be used in the multi-voice rhythm band shown in FIG. 11;

FIGS. 13 through show a multi-channel sound distribution system wherein FIG. 13 shows a system hav' ing photosensitive elements interposed between a plurality of input and output terminals, respectively;

FIG. 14 shows a housing for enclosing the system shown in FIG. 13; and

FIG. 15 shows means for selectively exposing the photo-resistive elements to a beam of light.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS:

Referring generally to FIGS. 1 through 8 of the drawings, the electronic sound synthesizer of the present invention includes a touch controlled generator having a plurality of electronic switches which are selectively actuable when a d.c. voltage source is applied thereto by shorting normally open conductive paths or touch plates interposed between the switches and the d.c. voltage source.

With apparatus in accordance with the invention, rather complex sounds having a wide range of frequency, amplitude, attack and decay, and overtone characteristics can be produced by modulating trains of predetermined wave shapes such as square, triangular, and/or sawtooth waves with the aforedescribed touch controlled generator and then putting the modulated outputs through a filter, reverberator, voltage controlled amplifier, and thereafter through a tone control circuit, power amplifier and speaker, as generally illustrated in FIG. 1, and more particularly shown in FIGS. 2 through 8.

The touch controlled generator also may be designed to generate certain sounds such as those of drums and multi-voice rhythm bands without the use of the variable d.c. voltage generators, filter and voltage controlled amplifier. Thus, as shown in FIG. 9, the synthesizer may be modified to generate drum sounds or, as shown in FIG. 11, it may be modified to generate sounds of a multi-voice rhythm band.

In the case of the drum sounds or multi-voice rhythm band generator, the outputs thereof may be fed directly into their own mixer and speaker to produce the desired audible sounds. As an alternative, the outputs thereof maybe mixed with a plurality of other inputs, reverberated and amplified, and then converted into audible sounds.

Referring to FIG. 2, the touch controlled generator is designed to generate two output signals, namely a gating pulse and a control voltage, when any one or more of the touchplates in the generator are shorted selectively. More particularly, the generator includes a plurality of electronic switches 15a, 15b l5n, respectively, connected to a +12 volt source through normally open conductor paths or plates 16a, 16b 16n, and a current limiting resistor 17. Each of the switches includes a field effect transistor FET 18, and NPN transistor 19, and a variable potentiometer 21.

Each of the conductive paths 16a, 16b 16n, has a bifurcated, flat conductor strip 23 connected to the current limiting resistor 17 and another fiat strip 25 positioned between the bifurcation 23 and connected to the gate electrode 27 of the FET 18. The conductor paths 16a, 16b 16!! may be formed on a fiat, dielectric substrate board 29 in a conventional manner so that the surfaces of the paths are in the surface plane of the dielectric board. Preferably, the dielectric gap between the conductors of each strip pair 23 and 25 is dimensioned so that the gap and the strip pair form a touch plate which can be shorted by a finger tip. The normal moisture on the surface of the fingers provides an excellent conducting path for short circuiting the dielectric gap.

Each of the gate electrodes 27a 27n of the FET is negatively biased with a 12 volt source through its current limiting resistor 33a, 33b 33n. A transistor network 35, connected to all of the switches in the manner shown, includes a PNP transistor 37 connected in a common emitter configuration with its emitter electrode connected to the +12 d.c. voltage source and its collector electrode connected to a bias resistor 39.

The drain electrodes 41a, 41b 41n of the FETs 18a, 18b l8n are respectively connected to the +12 volt d.c. voltage source through a resistor 43 shunted across the base and emitter electrodes of the transistor 37. The source electrodes 45a, 45b 45n of the FETs 18a, 18b l8n are respectively connected to ground through the base and emitter electrodes of the corresponding NPN transistors 19a, 19b 19n as shown. The collector electrodes of the transistors 19a, 19b l9n are connected to the respective variable potentiometers 21a, 21b 2ln through corresponding range limiting resistors 22a, 22b 22n. Currents through the switches 15a, 15b l5n pass through a summing bus 51 which forms a common junction at the outputs of all of the variable potentiometers 21a 2ln along with the range limiting resistors 22a 22n.

The touch controlled generator further includes one or more sets of cascade connected metal oxide semiconductor field effect transistors MOSFET 55 and 57 and a suitable operational amplifier 59 such as the integrated circuit type 741 operational amplifier/manufactured by Fairchild Semiconductor (Model uA74l) which responds to the current produced in the common bus 51 whenever any one of the touchplates is activated or touched. In this manner, a voltage is generated at the output terminal 60 the magnitude of which can be selected by touching one or more of the touchplates and adjusting the respective potentiometers. The generator includes another output terminal 61, connected to the collector electrode of the transistor 37 through a current limiting resistor 62 and a diode 63 connected as shown to provide a positive gating pulse when any touchplate is activated or touched.

More specifically, the aforementioned MOSFETs 55 and 57 and amplifier 59 are connected in the following manner: the common bus 51 is connected to the source electrode 65 of the MOSFET 55. The collector electrode 64 of the transistor 37 is connected via a resistor 68 to the gate electrode 67 of the MOSFET 55. The drain electrode 69 of the MOSFET 55 is connected to the gate electrode 71 of the MOSFET 57. The source electrode 73 of the MOSFET 57 is connected to a ground through a resistor 75. The voltage across the resistor forms an input to the negative input terminal of the amplifier 59. The drain electrode 74 of the MOSFET 57 is connected to a +12 volt d.c. voltage source. The positive input terminal of the amplifier 59 is provided with a positive d.c. bias by means of the voltage dividing resistors 81 and 82 connected between the +12 volt source and ground. The amplifier 59 includes the capacitors 83, 84 and 85 connected in the manner shown to bypass a.c. ripples or noise signals. The amplifier 59 itself is furnished with +12 volt and -l2 volt d.c. supplies in a conventional manner as shown. A storage capacitor'86 is connected from the output terminal 87 of the amplifier 59 to the gate electrode 71 of the MOSFET 57. This holds the output voltage at 87 at the previous level, even if the touching finger is removed, and until another switch is energized.

The bias voltage at the output terminal 87 of the amplifier 59 is set by a biasing network comprising resistors 91, 92, and 93. The amplitude range at the output of the amplifier 59 is controlled, that is, compressed or expanded, by varying the magnitude of the resistor 92. Similarly, the source electrode 65 of the MOSFET 55 is biased at a suitable level by a network comprising resistors 91, 96 and 97 connected in the manner shown and the bias level at the source electrode 65 is controlled by varying the resistor 96.

The touch controlled generator operates as follows: Each of the conductor paths or touch plates 16a, 16b 16n controls its associated FET 18a, 18b 18n and in turn the base drive of the corresponding NPN transistor 19a, 19b 19n. The FETs are normally turned off by applying the 12 volt source to the gate electrodes 27a, 27b 27n through the resistors 33a, 33b 33n. When a touchplate or path,for example 16a, is touched and activated, the bias on the gate electrode 27a changes from l2 volts to a value near zero. This causes a few micro amperes of current from the +12 volt source to flow through the touchplate 16a and the performers touching finger. This turns on the FET 18a. When the FET 18a is turned on, its drain electrode 41a draws current from the +12 volt source through the resistor 43 and the source electrode 45a. This current flows through the base electrode of the transistor 19a, and then to ground. When the current so drawn reaches approximately one milliampere, the transistor 37 turns on, and the voltage on its collector goes from --1 2 volts to approximately +12 volts. This is used as the gating pulse signal source. The gating signal is applied to the output terminal 61 through the resistor 62 and the diode 63. The diode 63 allows only the positive portion of the gating pulse to reach the output terminal 61. A resistor 99'is connected in the manner shown to provide a ground reference at the output terminal 61.

The gating signal at the collector of the transistor 37 is also used to turn on the MOSFET 55 through the resistor 68. The MOSFET 55 is designed to function as a switch so that it is either on or off. When one of the conductor paths 16a 16n is shorted, the voltage on the gate 67 of the MOSFET 55 changes from approximately --1 2 volts to volts. When a particular touch plate or path,for example 16a, is touched, the particular FET 18a and transistor 19a associated therewith are turned on and draw current through the corresponding range limiting resistor 22a and range controlling potentiometer 21a. This current is applied to the source electrode 65 of MOSFET 55 through the summing bus 51.

At this point, the MOSFET 55 is turned on by the action of the voltage change at the collector electrode of the transistor 37 and thereby effectively connects the summing bus 51 to the gate 71 of the MOSFET 57 via the drain electrode 69 of the MOSFET 55. The MOSFET 57 functions as a source follower for applying signals to the inverting, or the negative input of the amplifier 59.v Amplifier 59 acts as a current summing amplifier. MOSFET 57 acts as a high impedance negative input terminal to the amplifier 59 and allows the capacitor 86 to function as a storage memory, or holding device. The output voltage of the MOSFET 57, acting as the source follower, is developed across the resistor 75. The voltage at the output 87 of the amplifier 59 is controlled by the amount of current drawn in the current summing bus 51 which is in turn controlled by the potentiometer 21a through the particular switch connected to the touch plate 16a that has been activated by a touch of a finger. When the activating touch is removed, the voltage at the gate 67 of the MOSFET 55 goes negative and thereby turns it off. This breaks the connection from the gate 71 of the MOSFET 57 to the summing bus 51. The voltage output of the amplifier 59, however, remains at the value at which it was set when the MOSFET 55 was conductive, even after the performer has released his touch and the MOSFET 55 is turned off. This is made possible by the charge stored in the memory-capacitor 86.

The output, at 87, of the amplifier 59 is attenuated by the potentiometers 94 and 95. A switch 98 makes it possible to apply the output at terminal 87 directly to the potentiometer or through potentiometer 94 and then to the potentiometer 95 in order to attenuate it. The switch 98 functions to switch the output at the terminal 60 from one voltage range to another, while the potentiometers 21a, 21b 21n are used to vary the voltage range selectively, as the various conducting paths are touched. The control voltage and gating pulse generated in the aforedescribed manner are applied, respectively, to the wave generator B, the filter, and the voltage controlled amplifier through a suitable plug type connector, shown in the form of a block 64.

FIGS. 3 and 4 show wave generators A and B, respectively, which are voltage sensitive types. They are essentially identical except for certain differences in the processing of the incoming signals.

Referring to' FIG. 3, wave generator A includes a type 741 operational amplifier 101, which may embody integrated circuitry, used as a voltage integrator. it integrates a d.c. voltage at its positive terminal and stores the integrated voltage in a capacitor 105. When the voltage of the capacitor reaches the firing potential of the unijunction transistor UJT 103, it breaks down and conducts. This causes the capacitor 105 to discharge through a diode 135. This cycle is repeated to produce, at the output 107 of the amplifier 101, a sawtooth wave-form having a constant magnitude and a repetition rate depending on the input voltage level at the positive input terminal 109 of the amplifier 101. Generally, known integrators have the positive input terminal 109 grounded and a current applied to the negative input terminal 111. Thus such integrators are current integrators in that the output voltage is proportional to the negative integral of the current applied to the negative terminal. In contrast, according to the present invention, the input voltage is applied to the positive input 109 through a network consisting of the resistors 119, 121 and 122. In the case of wave generator A, a voltage may be applied through the jack 115 or, alternatively, the output plug 64 of the touch controlled generator may be coupled to the jack 100 so that the control voltage of the touch controlled voltage generator controls the output frequency of the wave generator A. The input voltages are attenuated through the resistors 119 and 121, and through the potentiometer 122. There is also a constant d.c. potential of +6 volts applied to the jack 115 at the point 120 from the +12 volts supply 123 as divided by the resistors 125 and 126. This potential of +6 volts produces a reasonably high audio frequency oscillation that remains constant ifnothing is plugged into the jack 115. If a plug is inserted in the jack 115, the +6 volts bias is disconnected.

An attenuated signal from the jack 115 or the terminal plug 100 is applied to the positive terminal 109 of the amplifier 101 and causes the amplifier 101 to operate. A current will flow to the negative terminal 111 from the amplifier output 107 through a diode 127 and a capacitor 105 so that an equivalent positive voltage appears at the negative terminal 111 across a resistor 129. When the voltage across the capacitor 105 reaches the firing potential of the UJT 103, the resistance between the emitter 131 and the base 132 of the UJT 103 becomes very small. When the UJT 103 fires, the capacitor 105 discharges through the UJT 103 and the diode 135. A resistor 136 provides a means for holding the base 132 of the UJT 103 at the ground potential. Thus the diode 135 serves the dual function of isolating the negative input 111 of the amplifier 101 from the resistor 136 to ground and of providing a path for the discharging capacitor 105. The diode 139 prevents the output 107 of the amplifier 101 from producing a large negative voltage if a negative voltage is applied to the positive input 109. The output 107 of the IC 101 is a positive going sawtooth wave whose repetition rate depends on the magnitude of the positive control input voltage at the positive input terminal 109. The amplitude of thesawtooth wave depends on the characteristics of the particular unijunction transistor UJT 103 used. The output at 107 is coupled to a potentiometer 142 via a dc. isolating capacitor 143. The potentiometer 142 is an attenuator which allows adjustment of the amplitude of sawtooth wave signals applied'to the input of a two-input, common emitter, inverting, transistor amplifier 154 of a conventional design, as shown. Various parameters of the in-.

verting amplifier are adjusted to invert the inputs and provide unity gain.

Another input of a square waveform also may be applied to the inverting amplifier, the square waveform being derived from the sawtooth wave by a conventional transistorized squaring network 170 of an appropriate design, as shown. The squaring network includes the attenuating potentiometers 173 and 174 so that the duty cycle and-the amplitude respectively of the output can be adjusted. Either or both of theoutputs of the squaring network 170 and the amplifier 101 may be applied to the inverting amplifier 154 by means of the switches 178 and 177, respectively, as shown.

The output of the inverting amplifier 154 at its collector 157 is RC coupled to an output jack 181. The output so obtained is internally applied via a conductor 201 to the input jack 203 (FIG. 4) of the wave generator B, so long as neither of the jacks are plugged.

While its normal function is to provide a suitable output for use in modulating the wave generator B, the wave generator A may also be used to produce pitches by connecting the plug 64 to the jack 100, then externally connecting its output directly to one of the mixer input jacks. In this way, the wave generator A can be modulated by the control voltage output of the touch controlled generator through the conductor via the jack 100.

FIG. 4 shows the wave generator B, which is essentially the same as the wave generator A except inthe way the input and output means are arranged. The wave generator B is the primary frequency (pitch) generator of the embodiment of the invention. It is designed with the potential of being frequency modulated by the output of either the wave generator A or that of the touch controlled generator or both.

The input at point 201 is attenuated by a potentiometer 205 to control the magnitude of the effect that the wave generator A has on the wave generator B. Thus the inputs to wave generator B are either from the output of wave generator A, through the conductor 201 or from the jack 203. Another input may be obtained from the jack 207, or through the conductor 60 from the jack 102 connected to the plug 64 which, in turn, is connected to the touch controlled generator shown in FIG. 2. These inputs are summed through the resistors 119' and 121' and fed to the positive input terminal 109' of a type 741 operational amplifier 101'. (Reference numerals are primed where they are identical with those of FIG. 3).

As shown in FIG. 7A and 7B, the output of the wave generator A is a'train of inverted sawtooth and square waves, respectively, depending on which of the switches 177 and 178 is closed and which is open. Given a dc. voltage of a fixed amplitude at terminal 109', the wave generator B will produce a square wave train as shown in FIG. 7C, a triangular wave train, FIG. 7D, and a sawtooth wave train, FIG. 7E, at potentiometers 174', 217 and 142, respectively.

The output of the wave generator B at 107' is a train of waves whose frequency varies according to the voltage present at the input 109, which is obtained from the wave generator A through the conductor 201, or from a source of voltage external to the system through either jack 203 or 207, or from a control voltage from the touch controlled generator through the jack 102 and the conductor 60, or from any combination of these sources. Thus, if the output of the wave generator As in the oscillator A, a squaring network 170' is connected to the output of the operational amplifier 101'. This produces a train of square waves (FIG. 7C) at its output 171 of the same frequency as the train of sawtooth waves at the output 107 and the duty cycle T of the waves can be varied by a potentiometer 173 in the squaring network. The output of the squaring network is RC coupled to the summing point 158 of the inverting amplifier 154 through an attenuating potentiometer 174' and the resistor 210. Two RC networks formed by a resistor 211 and a capacitor 212 and a resistor 214 and a capacitor 215, respectively, function as an integrator to convert the square wave train, such as FIG. 7C, into a triangular wave train, FIG. 7D. The triangular wave train is relatively low in harmonic content and is attenuated by a potentiometer 217 and applied to the summing point 158' through a resistor 219. Accordingly, the inverting amplifier 154' functions as the summing amplifier of the three aforementioned input waves.

The output voltage of the inverting amplifier 154' is proportional to the sum of the square wave input as attenuated by the potentiometer 174', the triangular wave input as attenuated by the potentiometer 217, and the sawtooth wave input as attenuated by the potentiometer 142'. If, for example, the train of rectangular waves, as shown in FIG. 7G, from the wave generator A through the conductor 201 is applied to the input of the wave generator B, and if the attenuators 174' and 217 are set to full attenuation, the output wave form at the conductor 225 would appear in the form shown in FIG. 7F, but inverted. Thus, the output of the wave generator B at the jacks 221 and 223 may be an inverted sawtooth, a square or triangular wave individually, or any additive combination of the three, each having the same frequency. The output of the inverting amplifier 154' is RC coupled to the jacks 221 and 223. If the jack 223 is not plugged, then the output of the oscillator B is directly connected through the conductor 225 to the input of the voltage controlled filter, shown in FIG. 5. With a control voltage wave train of the shape shown in FIG. 7I from the output conductor 60 of the touch controlled generator applied to input conductor 60' of the wave generator B through the plug 64 and the jack 102 and with the attenuators 142' and 217 adjusted so as to null the sawtooth and triangle waves generated by the wave generator B, the output at terminal 225 is in the form shown in FIG. 7H. For the sake of clarity, FIGS. 7H and I assume the output 60 of the touch controlled generator and are the only voltage appearing at the input to the wave generator B.

However, it is evident from the above description that the output of the wave generator B, at the jacks 221 and 223, is a composite of the trains of sawtooth waves, triangular waves, and rectangular waves of varying amplitudes, and the frequency of the output is largely dictated by the amplitude of the control voltage from the touch controlled generator.

FIG. illustrates the voltage controlled filter of an RC type wherein a capacitor 231 is used as the capacitive element, and one half of a dual field effect transistor FET 233a is used as the variable resistor. The capacitor 231 and the FET 233a may be connected as shown in FIG. 5 to act as a low pass filter, or

in the manner shown in the inset as a high pass filter. The various parameters of the filter are adjusted to provide a 6 dB per octave rolloff.

As shown, the output signal from the jack 223 of the wave generator B, FIG. 4, is applied as the input signal at the jack 237. The input signal is attenuated by a voltage divider network formed of resistors 239 and 241 to a level of approximately 0.1 volts. It is then applied to the FET 233a, which acts as a variable resistor having a value which is determined by the voltage at its gate 243. The high frequency components are filtered out through the capacitor 231 connected in the manner shown. A 741 type integrated circuit operational amplifier 242 is provided to amplify the resulting signal across the capacitor 231. Another input, unfiltered, is applied to the negative terminal of the amplifier 242 via the jack 248 and the resistor 249. The resistors 251 and 253 set the feedback characteristics around the amplifier 242 to compensate for the losses through the voltage divider network resistors 239 and 241, the FET 233a and the capacitor 231.

The drive voltage for the gate 243 of the FET 233a is derived from the gating voltage output at the conductor 61 of the touch controlled generator applied through a filter control network 241, a pair of 741 type integrated circuit operational amplifiers 243 and 245, and the other half of the FET 233, namely FET 233b, as explained in detail below.

The resistors 251 and 253 form a voltage divider to bias the positive input of the amplifier 245 to approximately 0.l volts. A small current flows through the FET 233b, and the voltage at its gate 255 is varied by the input 257 of the amplifier 245, so that approximately 0.l volts appears across the FET 233b at point 257. Thus the FET 233b acts as a variable resistor whose value is set by the current flowing therethrough. A larger current will cause the amplifier 245 to change the voltage at the gate 255 of the FET 233b so that the resistance from the point 257 to ground through the FET 233b decreases. The current through the FET 233b is set at some minimum value, for example approximately 1.0 microamperes, by the resistor 261. Additional current can flow through the FET 233b, resistor 263 and diode 265 from the output of the amplifier 243.

The amplifier 243, used in conjunction with the resistors 267 and 269, functions as a unity gain, inverting voltage amplifier under the control of the filter input control circuitry 241. The resistance across the source and drain terminals 275 and 277 of the FET 233a is inversely proportional to the output voltage amplitude of the amplifier 245 applied to the gate electrode 243. This makes the cutoff frequency of the filter output at 307 directly proportional to the voltage atthe input 271 of the amplifier 243. This is illustrated in FIG. 8A.

The filter input control circuitry is designed to provide somewhat less than unity gain and is a source follower type amplifier which changes the rise and fall speeds of a square wave input at the filter control jack 279 and applies the changed square wave at its output terminal 281. A square wave of approximately 10 volts is applied to the filter at the jack 279 or, alternatively, the pulse train output at the conductor 61 of the touch controlled generator, FIG. 2, is applied to the jack 279 as shown. A series network of resistors 283, 285, and

the diode 287, in conjunction with the capacitor 295 connected in the manner shown, determines the fall time of the outputof the control circuitry. Another similar series network of resistors 289 and 291 and the diode 293 in conjunction with the capacitor 295 determines the rise time. When the potentiometers 283 and 289 are set at their minimum values, the rise and fall of the waveform at the output terminal 281 are sharp as shown in FIG. 88, whereas when they are set at their maximum values, the rise and fall are slow as shown in FIG. 8C. An FET 297 is used as a follower to allow a relatively low impedance output without loading the capacitor 295. The resistor 299 functions as a trimmer and allows the output voltage at the terminal 281 to be set to zero when the input voltage at the jack 279 is zero.- A resistor 301 is used to connect a -l2 volt negative power supply to the trimmer 299.

The aforedescribed voltage controlled RC filter alters the frequency content of the input signal at the input terminal 237 in response to a changing low frequency voltage which may be applied at the jack 279 or to the gating pulse of the touch controlled generator at the conductor 61 shown in FIG. 2. The rise and fall times of the low frequency voltage are individually adjustable by varying the potentiometers 283 and 289. FIG. 8D shows the effect on the wave shape of the output of the filter at the terminal 307, with a square wave input at the conductor 237, as affected by the voltage at the output 281 (FIG. 8E) of the input control circuitry 241 of the filter.

The filter may be either a high-pass or low-pass device as previously described. Inset 135 illustrates the reversal of the positions of the FET 233a and the capacitor 231 necessary for implementing the highpass filter version. In either embodiment, the filter will track the voltage vs. frequency relationship in a predictable manner as illustrated in FIG. 8A. Briefly restated, the gating pulse from the output conductor 61 of the touch controlled generator is applied to the filter input control circuitry 241 through the jack 279 to control the nature of the filtering action of the voltage conmanner shown to control, respectively, the rise and fall time of the voltage across the capacitor 321. A field effect transistor FET 335 is connected to perform the function of a follower amplifier and to provide a relatively low impedance output from the signal that appears across the capacitor 321. The trimming resistor 337 and a resistor 339 connected to the l2 volts terminal of a power supply in the manner shown provide means for adjusting the potential at output point 341 of the FET 335 to 0 volts when the input 343 of the envelope control circuitry is 0 volts. A varying voltage, appearing at the base of the transistor '345 in response to the envelope control signal, varies the amount of current drawn by the transistor 345. Thus the transistor 345 acts as a current source the magnitude of which depends ultimately on the voltage at the input 343 of the envelope control circuitry 317. This current flows through a resistor 347 and the transistor 345, and is applied to the first amplifier stage 313.

The first amplifier stage includes a pair of transistors 351 and 353 connected as a differential circuit to which the output of the filter in FIG. 5 is coupled through a capacitor 359 after an attenuation by the resistors 355 and 357. The base electrodes of the transistors 351 and 353 are connected, respectively, through the base resistors 361 and 363 to a common junction 365. An AC voltage at the common junction 365 is bypassed to ground via the capacitor 367. The DC voltage at this junction is determined by two voltage divider networks, the first of which includes the resistors 371 and 373 that divide the +12 volts supply to provide approximately +6 volts at the point 375, and the second of which includes the resistors 376 and 377 which reduce the 6 volt potential to +4.5 volts for application to the base electrodes of the transistors 351 and 353. The collectors of the two transistors 351 and 353 are connected to +6 volts through the collector retrolled filter. The range of change in the frequency q response is made adjustable by a potentiometer 303 connected to the output of the filter control circuitry 241 which, in turn, controls the output at the conductor 307 of the filter in FIG. 5. This output is then applied to a voltage controlled amplifier shown in FIG. 6.

The voltage controlled amplifier is designed so that its gain can be varied from zero, i.e., zero signal output, to approximately unity. Briefly, it includes first and second amplifying stages 313 and 315 and envelope control circuitry 317. The output from the filter is applied to the first stage whose gain is modulated by the gating pulses from the touch controlled generator, or any train of low frequency pulses, via the envelope control circuitry 317. The first stage is essentially a low level and differential type amplifier. The output of the first stage is amplified by the second stage for subsequent application to a mixer.

More specifically, the envelope control circuitry 317 may be of the type which is similar to the filter input control circuitry 241 of the voltage controlled filter in FIG. 5. It includes a capacitor 321 and a pair of series networks of resistors 323, 325, 327 and 331, and the diodes 329 and 333, respectively, connected in the sistors 379 and 381, respectively. The emitters of the two transistors are tied together and connected to the collector of the transistor 345.

. The amplifier operates as follows: The differentially connected transistors 351 and 353 divide the control current flowing through the common emitter junction 370 from the collector of the transistor 345 to the two collectors 383 and 385 in proportion to the difference in voltage between the two base electrodes 387 and 389. Therefore, the magnitude of the differential currents that flow in the two collector leads 383 and 385 is proportional to the product of the input voltage between the two base connections 387 and 389 and the current flowing in the common junction 370 via their emitters. The current at the common junction 370 is modified by the gating pulse voltage applied at the input 343 of the envelope control circuitry 317. The signal at the collector electrodes 383 and 385 of the transistors 351 and 353 could be used directly as the output of the amplifier and applied to the mixer. However, the signal amplitudes at the collector electrodes 383 and 385 are at a very low level. Accordingly, a second amplifying stage 315 having a pair of transistors 391 and 393 is used to amplify the low level signals. The two transistors are provided with resistors 395 and 397 connected to the emitter electrodes thereof to improve the linearity and to reduce the noise and distortion of the signal being amplified. The two resistors are 

1. An electronic sound generator comprising: means for generating a train of oscillating waves; a plurality of circuit means for generating d.c. signals of different amplitudes; a plurality of means for selectively actuating said plurality of circuit means for generating corresponding d.c. signals; means for summing the output of the d.c. signals generated by said actuated circuit means; means responsive to the sum of said generated d.c. signals for generating a control voltage, the amplitude of which varies in accordance with the variation in the amplitude of the sum of said generated d.c. signals; means for modulating said train of oscillating waves with said control voltage; and means for converting said modulated train of oscillating waves into audible sounds.
 2. The generator according to claim 1, including a d.c. voltage supply wherein said plurality of circuit means include a plurality of electronic switches and means for biasing said switches so that they are normally turned off; said plurality of actuating means include a plurality of touch plates, each plate having a dielectric means, first and second conductors spaced from each other by said dielectric means, said first conductor connected to said d.c. voltage supply and said second conductor connected to a corresponding one of said switches wherein said dielectric means is dimensioned so that it can be bridged by a fingertip to provide a conducting path for said d.c. voltage supply to actuate the corresponding switch.
 3. The generator according to claim 2, wherein each of said switches includes a field effect transistor connected to the corresponding one of said touch plates so that said field effect transistor is actuatable by said d.c. voltage supply when the corresponding one of said dielectric gaps between the first and second conductors is bridged over conductively, a transistor responsive to the actuation of said field effect transistor for generating the d.c. signal, and a variable impedance means connected to the output of said transistor for setting the amplitude of said d.c. signal.
 4. The generator according to claim 1, wherein said responsive means includes first and second metal oxide silicon field effect transistors and an operational amplifier connected in series, the source electrode of said first field effect transistor being connected to the said summing means, and the drain electrode of said first field effect transistor being connected to the gate electrode of said second field effect transistor, the source electrode of said field effect second transistor being connected to said operational amplifier.
 5. The generator according to claim 4, wherein said responsive means further includes a potentiometer interposed between said d.c. supply voltage and the source electrode of said first metal oxide silicon field effect transistor for controlling the amplitude of said control voltage.
 6. The generator according to claim 4, wherein said responsive means includes a capacitor shunted between the input terminal of said second field effect transistor and the output terminal of said operational amplifier for maintaining the control voltage output at the previous level between successive actuations of said switches.
 7. The generator according to claim 2, including a common emitteR transistor stage interposed between the drain electrodes of said field effect transistors in said plurality of electronic switches and unipolar conducting means connected to the collector electrode of said common emitter transistor stage to respond to the actuation of any said plurality of circuit means for generating a positive gating voltage, wherein said converting means is adapted to respond to said gating voltage for synchronizing the operation thereof to the actuation of said circuit means.
 8. The generator according to claim 7, wherein said means for generating oscillating waves includes a first wave generator for generating trains of first sawtooth waves, a second wave generator for generating first square waves, in response to said first sawtooth waves, and means for sending out said sawtooth waves and square waves selectively, and a third wave generator responsive to the output of said first and second wave generators and said control voltage, for generating trains of second square waves, triangular waves and second sawtooth waves, and means for combining said trains of waves into a single train of composite waves.
 9. The generator according to claim 8, wherein said converting means include filtering means responsive to said gating voltage for filtering said train of composite waves.
 10. The generator according to claim 9, wherein said filtering means includes a variable RC impedance network and first operational amplifier connected in a series, connected to the output of said third wave generator to respond to said train of composite waves, and a control circuitry responsive to said gating voltage for generating a control signal for modifying the filtering characteristics of said variable RC impedance network.
 11. The generator according to claim 10, wherein said control circuitry includes a filter input control circuitry having a field effect transistor, first and second passive impedance networks interposed between the gate electrode of said field effect transistor and the gating voltage output terminal, and a grounded capacitor connected to the gate electrode, said networks being adjustable to control the rise and fall time of said control signal, respectively.
 12. The generator according to claim 11, wherein said first and second passive impedance networks are connected in parallel and respectively include a variable resistor for controlling the amplitude range of said control signal and a diode connected in series, and wherein the diode of said first network is poled opposite to that of said second network.
 13. The generator according to claim 12, wherein said control circuitry includes means for setting the control signal to zero amplitude when the gating voltage is set at zero.
 14. The generator according to claim 13, including second and third operational amplifiers connected in series and responsive to the output of said filter input control circuitry, said variable RC impedance network including the first half of a dual field effect transistor and a capacitor, the other half of said dual field effect transistor being shunted by the inverting input of said third operational amplifier to ground.
 15. The generator according to claim 14, wherein the source electrode of said first half of the dual field effect transistor is connected to the output of said second wave generator, the drain electrode thereof is connected to the input of said first operational amplifier, the gate electrode thereof is connected to the output of said third operational amplifier, and a capacitor is shunted between the input to said first operational amplifier and ground, for providing a low pass filtering action.
 16. The generator according to claim 14, wherein the capacitor of said variable impedance is interposed between the input terminal and said first operational amplifier, said first half of said dual field effect transistor is connected between the input of said first operational amplifier and ground, for providing a high pass filtering action.
 17. The generator according to claim 10 wherein said converting means includes an envelope control circuitry responsive to said gating voltage for providing a high and a low pass filtering action, selectively, and a voltage controlled amplifier responsive to the output of said envelope control circuitry for shaping the attack and decay characteristics of the modulated and filtered signal from the output of said filtering means.
 18. The generator according to claim 17, wherein said envelope control circuitry includes a field effect transistor, a first and second impedance network connected in parallel and interposed between said field effect transistor for controlling the output of said field effect transistor proportional to the amplitude of the gating voltage, and means for setting the output of said field effect transistor to zero voltage when said gating voltage is set at zero.
 19. The generator according to claim 17, wherein said voltage controlled amplifier circuitry includes first and second amplifying stages connected in series, and said envelope control circuitry controlling the attack and decay characteristics of the amplification provided by said first amplifying stages.
 20. The generator according to claim 1, wherein said converting means includes a mixer for combining the outputs of said modulating means, and a series network of reverberator, tone control circuit, power amplifier and speaker responsive to the output of said mixer.
 21. An electronic sound system comprising means for supplying d.c. voltage; a plurality of generators for producing a plurality of decaying a.c. signals, said generators including a plurality of networks having twin - T tuned oscillators connected to inverting amplifiers, a keyboard means having a plurality of conductive paths connected between said generators and said d.c. voltage supply means respectively, dielectric means interposed in said plurality of conductive paths, wherein shorting of selected ones of said conductive paths, energizes corresponding ones of said generators, means for converting said a.c. signals into audible sounds, and a disturbance generator connected between said T tuned oscillators and said conductive paths, wherein aid disturbance generator includes a field effect transistor connected to a conductor pair of said keyboard means for setting said field effect transistor in a normally non-conducting state, and a transistor coupled to said field effect transistor for supplying an enabling trigger signal to one of said T tuned oscillators upon actuation of said field effect transistor when said pair of conductors is touched thereby supplying enabling power to said field effect transistor.
 22. The system according to claim 21, wherein each of said plurality of conductive paths includes, first, relatively flat, elongated conductive strips having bifurcated projections and, second, a relatively flat, elongated conductive strip positioned between said bifurcated projections, and a dielectric material interposed between said first and second strips, wherein the plurality of corresponding pairs of conductive strips are provided with exposed conductive surfaces which can be shorted by touching.
 23. The system according to claim 22, wherein each of said plurality of generators includes a ringing circuit continuously tunable over a range of frequencies and one of said disturbance generator for applying a trigger signal to said ringing circuit when a pair of corresponding strips is shorted by the touch and capacitive means shunted across ringing circuit for slowing the return of said disturbance generator to its off state after the touch across the conductor pair is removed.
 24. The system according to claim 21, further including means for enabling said transistor to turn off gradually when the touch is removed and the field effect transistor is thereby de-energized.
 25. The system according to claim 26, wherein said preventing means includes a capacitor connected across said transistor.
 26. The system according to claim 21, further including an inverting amplifier network coupled to said ringing network, said amplifier network having means for introducing high and low distortion to the output of said collector selectively.
 27. The system according to claim 26, wherein said high and low distortion introducing means includes a series network of a diode and a switch shunted across said amplifier network.
 28. The generator according to claim 27, further including an output amplifier stage coupled to the outputs of a plurality of inverting amplifiers.
 29. The system according to claim 21, wherein said plurality of generators include a plurality of ringing networks adapted to generate damped ringing signals of different frequencies respectively for simulating multi-frequency drum signals, a plurality of local mixers for combining the outputs of a predetermined number of said ringing networks, and a final mixer for combining all of the outputs of said plurality of local mixers, and a converting means for producing sounds responsive to the output of the final mixer.
 30. The system according to claim 29, wherein each of said plurality of generators includes a ringing network and a disturbance generator for enabling said ringing network to generate a damped ringing oscillatory signal in response to the voltage applied thereto through said keyboard means upon touching and shorting of corresponding pairs of said conductor paths.
 31. The system according to claim 30, wherein said ringing circuitry includes means for tuning it to a different frequency, and said disturbance generator includes a field effect transistor circuit connected in series, and capacitive means shunted across said transistor for slowing the return of said transistor to the off state after the touch across the conductor pair is removed. 